Given,
Refractive index of ice = 1.31
Refractive index of rock salt = 1.54
We know,
Refractive index = SpeedoflightinvacuumSpeedoflightinthemedium
Refractive index of rock salt with respect to ice =refractiveindexofrocksaltrefractiveindexofice
Refractive index of rock salt with respect to ice =1.541.31 =1.175
Hence, the Refractive index of rock salt with respect to ice is 1.175